Neurons of the dorsal column postsynaptic tract (DCPST) were retrogradely labelled with HRP. Most cells were found in lamina IV. Counts of these neurons showed an average of 41.5 neurons in each rostrocaudal millimeter of the dorsal horn. Antidromically identified and physiologically characterized DCPST neurons were intracellularly stained. Their dentritic arbors were rostrocaudally elongated (c. 1450 Mum) but relatively compressed mediolaterally (c. 450 Mum). Several cells were found to issue varicosity-bearing axon collaterals within and below their dendritic territories. Immunocytochemical studies revealed small interneurons in lamina III that were immunoreactive for enkephalin (ENK). Their perikarya were 10-15 Mum and their sparse dendritic arbors coursed rostrocaudally and dorsally. Neurons with apparently identical morphology were intracellularly stained with HRP. They all responded exclusively to activation of low threshold mechanoreceptive primary afferents with A beta axons. ENK neurons were found whose perikarya, proximal dendritic arbors, ultrastructure, and synaptic connectivity resembled lamina IIb islet cells.